Drilling fluids (muds) are normally used in drilling oil and gas wells. These fluids are used to maintain pressure, cool drill bits, and lift cuttings from the holes as the well is being drilled. Drilling fluids vary greatly in composition depending upon specific requirements of the well being drilled as well as geological considerations. However, drilling fluids typically fall into the class of aqueous formulations or oil-based formulations.
Early oil-based drilling fluid formulations that are no longer used were typically comprised of the following ingredients: oil (generally No. 2 diesel fuel), emulsifying agents (alkaline soaps and fatty acids), wetting agents (dodecylbenzene sulfonate), water, barite or barium sulfate, (weighting agent), asbestos (employed as viscosification agent) and/or, amine-treated clays (also as viscosification agent). These oil-based drilling fluid formulations were generally formulated based primarily on amount of barite added. For example, such a typical drilling fluid could range in specific gravity from about 7 pounds per gallon up to 17 pounds per gallon or even greater. This variation in specific gravity is primarily controlled by the amount of barite added.
Oil-based drilling fluid formulations perform adequately in a number of applications, primarily those where the use of oil-based drilling fluids is dictated by the lack of stability of the formation in which drilling is taking place. For example, in various types of shale formation, the use of conventional water-based fluids can result in a deterioration and collapse of the shale formation. The use of the oil-based formulations circumvents this problem. However, traditional oil-based drilling fluid formulations also have some significant disadvantages. One disadvantage is that the incorporation of asbestos or asbestos fines can result in significant health problems, both during the fluid formulation and potentially during the subsequent use of such formulations. Therefore, in recent years there has been a strong push to reduce the level of asbestos used in such formulations or to eliminate the use of asbestos completely. The use of substitutes for asbestos as viscosity enhancing agents in such application has not been universally successful by virtue of the fact that the replacement must maintain adequate viscosities under the drilling conditions which can involve high temperature and high shear conditions.
As noted in U.S. Pat. No. 4,425,463, there has been a substantial need for a drilling fluid which would exhibit good performance at high temperatures in water sensitive formations. Past experience has shown that oil-based drilling fluids can provide good performance in water sensitive formations, and the state of the art systems can perform well at temperatures of up to about 350° F. (177° C.). In cases where the viscosity of conventional oil-based drilling fluids break down during drilling operations additional viscosifier is added to the drilling fluid being circulated into the well. In other words, the problem of viscosity loss during drilling is traditionally circumvented by the addition of more viscosifier to the drilling fluid being circulated into the well. While this solution is adequate at moderate temperatures the degradation of the viscosifier can be so rapid at high temperatures, such as those encountered in drilling geothermal wells and natural gas wells, that cost of utilizing the amount of additional viscosifier required can become cost prohibitive. There is accordingly a need for oil-based drilling fluids that can maintain their viscosity and gel strength at temperatures of 400° F. (204° C.) or even higher.
U.S. Pat. No. 4,525,522 describes an approach to viscosification of oil-based drilling fluids which permits the substitution of latices of sulfonated ionomers for asbestos fines and amine clays. These resulting polymer-modified drilling fluids are reported to display improved low temperature rheological properties which include improved gel strength at up to temperatures of 400° F. (204° C.) and higher, based on tests conducted for 16 hours at such temperatures.
U.S. Pat. No. 4,525,522 more specifically discloses latices of sulfonated thermoplastic polymers which function as viscosification agents when added to oil-based drilling fluids which are the fluids used to maintain pressure, cool drill bits and lift cuttings from the holes in the drilling operation for oil and gas wells. The sulfonated thermoplastic polymer of these latices contain about 5 to about 100 meq. of sulfonate groups per 100 grams of the sulfonated thermoplastic polymer, wherein the sulfonated groups are neutralized with a metallic cation or an amine or ammonium counterion. U.S. Pat. No. 4,525,522 further reports that a polar cosolvent can optionally be added to the mixture of oil drilling fluid and sulfonated polymer, wherein the polar cosolvent increases the solubility of the sulfonated polymer in the oil drilling fluid by decreasing the strong ionic interactions between the sulfonate groups of the sulfonated polymer.
U.S. Pat. No. 4,447,338 discloses that sulfonated EPDM is very effective as a viscosifier for oil-based drilling fluids. U.S. Pat. No. 4,447,338 more specifically discloses an oil base drilling fluid which comprises: (a) an organic liquid selected from the group consisting of a diesel fuel, kerosene, fuel oil and crude oil; (b) about 1 to about 10 parts by weight of water per 100 parts by weight of the organic liquid; (c) about 20 to about 50 lb/bbl of at least one emulsifier; (d) weighting material of sufficient quantity necessary to achieve the desired density; and (e) about 0.25 to abut 2 lb/bbl of a water insoluble neutralized sulfonated elastomer, said neutralized sulfonated polymer elastomer having about 5 to about 30 meg. of sulfonate groups per 100 grams of the neutralized sulfonated polymer elastomer, said neutralized sulfonated elastomer being derived from an elastomeric polymer selected from the group consisting of EPDM terpolymers and butyl rubber, said EPDM terpolymers having a number average molecular weight of about 10,000 to about 200,000 and said butyl rubber having a Staudinger molecular weight of about 20,000 to about 500,000.
U.S. Pat. No. 4,425,463 discloses the use of mixtures of sulfonated thermoplastic polymers and amine-treated clays as viscosification agents for utilization in oil-based drilling fluids. U.S. Pat. No. 4,425,463 muse specifically discloses a oil-based drilling fluid which comprises: (a) an organic liquid immiscible with water; (b) about 1 to about 10 parts by weight of water per 100 parts by weight of the organic liquid; (c) about 20 to about 50 lb/bbl. of emulsifier; (d) weighting material necessary to achieve the desired density; (e) about 0.25 to about 4.0 lb/bbl. of water insoluble neutralized sulfonated thermoplastic polymer having about 5 to about 100 meq. of sulfonate groups per 100 grams of the neutralized sulfonated thermoplastic polymer; and (f) about 1 to about 10 lb/bbl. of an amine-treated clay.
U.S. Pat. No. 5,021,170 discloses an invert emulsion drilling fluid comprising a liquid oleaginous medium, water, an emulsifier and a gellant comprised of sulfonated ethylene/propylene/5-phenyl-2-norbornene terpolymer and an organophilic clay. U.S. Pat. No. 5,021,170 more specifically reveals a oil-based drilling fluid comprising: (a) a liquid oleaginous phase; (b) a polar liquid phase, said oleaginous phase being present in an amount of from about 30 to about 98% by volume of the liquid phase, said polar liquid phase being present in an amount of from about 2 to about 70% by volume of the liquid phase; (c) an emulsifier; and (d) a gellant comprising a sulfonated ethylene/propylene/5-phenyl-2-norbornene terpolymer having a number average molecular weight of about 5,000 to about 300,000; and an organophilic clay comprising the reaction product of an organic onium compound and a smectite clay, the weight ratio of said organophilic clay to said terpolymer being from about 6:1 to about 20:1, said gellant being present in an amount sufficient to viscosify said oleaginous medium to the desired degree.
As has been noted, it is frequently important for the viscosification agents employed in drilling fluids to provide the desired level of viscosity at high service temperatures for extended periods of time. It is also critical for drilling fluids to provide the desired service characteristics, such as maintaining pressure, cooling drill bits and to lift cuttings from the hole being drilled, without causing formation damage. For instance, formation damage can be caused by organoclays used in conventional drilling fluids plugging the pores of rock formations. Good filtration behavior is another characteristic that it is desirable for drilling fluids to exhibit. A low level of the drilling fluid being lost in the rock formation is indicative of good filtration behavior. Finally, it is desirable for the viscosification agent to provide the desired increase in viscosity at a relative low concentration in the drilling fluid. There has been a long felt need in the well drilling industry for an improved drilling fluid that exhibits all of these desirable characteristics.